Optical sensors are used in a large number of applications such as scanning or digitising systems, optical pointing devices and imaging devices. Optical sensors generally operate by detecting electromagnetic radiation and producing an electrical signal that corresponds to the intensity of the electromagnetic radiation impinging on the optical sensor. Multiple optical sensors are generally used and are often geometrically positioned in arrays with individual optical sensor corresponding to a respective pixel of the array.
Each optical sensor typically comprises a photodetector or photodetecting device, such as a photodiode or phototransistor, for converting electromagnetic radiation into an electrical signal. One specific type of optical sensor further includes an integrating circuit associated with the photodetecting device. In such type of optical sensor, the photodetector generates a photocurrent the value of which is a function of the electromagnetic energy striking the photodetector and this photocurrent is integrated over time, i.e. converted, by the integrating circuit to produce an output voltage.
One already knows optical sensors which include a passive integrating circuit. In such a passive type optical sensor, the photodiode used as photodetector (as well as its associated junction capacitance and attached parasitic capacitance) is initially biased to a high reverse voltage. The photodiode generates a photocurrent which discharges the capacitance, thereby causing the voltage to decrease. The output voltage for this type of optical sensor is generally non-linear with respect to the integrated charge since the diode capacitance is a function of the diode voltage. A further disadvantage of the above passive type optical sensor resides in the fact that the integrating capacitance (defined in this case by the photodiode capacitance and the parasitic capacitance) is determined primarily by the photodiode size. Accordingly, sensitivity cannot be increased by increasing the photodiode size since the capacitance will increase approximately proportionally.
One additionally knows active-type optical sensors comprising a photodetector (typically a photodiode) coupled to an active integrating circuit basically comprising an operational amplifier having a non-inverting input coupled to a reference voltage, an inverting input coupled to the photodiode and an output, an integrating capacitor being connected across the operational amplifier output and inverting input.
An improved optical sensor and integrating circuit are proposed in U.S. Pat. No. 6,031,217. According to this patent, the optical sensor comprises a photodetector (preferably a photodiode) having an output proportional to the intensity of the electromagnetic radiation striking the photodetector and an active integrating circuit coupled with the output of the photodetector for integrating the photodetector output over time to generate an output electrical signal. This integrating circuit basically comprises an operational amplifier having a non-inverting input coupled to a reference voltage, an inverting input and an output, the photodetector being coupled to the amplifier inverting input and to the reference voltage. The integrating circuit further comprises an integrating voltage storage device, i.e. a capacitor, connected across the operational amplifier output and inverting input for storing an accumulated electrical signal from the photodetector. Switching circuitry is further provided for controlling the timing of the active integrating circuit.
According to the above patent, the integrating circuit is operated in such a way as to maintain a photodiode biased at a level close to zero, thereby reducing any associated dark currents. By resetting the output of the integrating circuit to near zero, prior to the start of each integrating cycle, the dark level output of the circuit is sufficiently close to ground that dark level baseline restoration is not normally required. The result is a lower pixel-to-pixel dark level variation and also a reduction in dark current errors at elevated temperatures. In addition, for variable temperature situations, the dark level currents may be reduced and the sensitivity increased as compared to comparable techniques using reversed biased photodiodes.
As mentioned above, the output of the integrating circuit is reset to near zero prior to the start of each integrating cycle. The voltage across the integrating capacitor is therefore null during reset, i.e. below the threshold voltage of a standard MOS transistor. Accordingly, one disadvantage of the solution disclosed in U.S. Pat. No. 6,031,217 resides in the fact that it is not possible to realize the integrating capacitor as a standard MOS transistor, i.e. a MOS transistor having source and drain terminals connected together. Poly or metal layers must therefore be used, which are very area-inefficient.
Maintaining a zero bias voltage across the photodiode indeed has the advantage of reducing any associated dark currents as described in the above patent. However, the photodiode “needs” a large reverse bias across it to be fast and efficient in its collection of photon-generated current. For applications requiring short response time and high collection efficiency, the solution proposed in U.S. Pat. No. 6,031,217 is therefore not adequate.
Another disadvantage of the above solution resides in the fact that the optical sensor requires two separate control lines (designated OSZERO and RUN in U.S. Pat. No. 6,031,217) for zeroing the operational amplifier and for controlling the timing of the integrating circuit. It is generally preferable to keep the number of external lines at a minimum so as to reduce the number of connections to each optical sensor which inevitably necessitate area on the chip.
Accordingly, it is an object of the present invention to provide an optical sensor comprising a photodetector and an integrating circuit which makes it possible to use a standard MOS component as integrating capacitor, thereby avoiding extra process steps, such as the formation of a second poly or metal layer and using die area in a more efficient manner.
A further object of the present invention is to provide a solution that allows to maintain a relatively large reverse bias across a photodiode so as to ensure a fast and efficient collection of photon-generated charges.
Still another object of the present invention is to minimize the number of external connections of the optical sensor in order to avoid wasting area in applications necessitating multiple optical sensors, such as in the case of optical sensing arrays.
Yet another object of the present invention is to provide an optical sensor that may be operated at low voltages and that exhibits low power dissipation.
Another object of the present invention is to provide a solution that meets several other requirements and constraints such as, amongst other things, large output signal swing, high integration gain, high amplifier gain in both modes (integration/run and reset) for accurate reset and integration, amplifier stability, low output impedance to drive the followings signal processing stages, output signal level such that single-pass-transistor switches can be used in following signal blocks, minimal parasitic capacitance at photodiode input, and area efficient layout.